scaling images in libgdx only once

Question

In my android game, I am using images of fixed resolution lets say 256x256. Now for different device screens, I am rendering them by calculating their width and height as appropriate for that device.

Assume that on galaxy note2 I calculated width=128 and height=128... similarly for different devices, width and height will vary.

This is how I created texture..

....
imageTexture = new Texture(...);
....

in render()..

.... 
spriteBatch.draw(imageTexture,x,y,width,height);
....

So, every time when I call draw() method, does libgdx/opengl scale image from 256x256 to 128x128, which I think, yes!

Is there any way to tell opengl/libgdx to calculate all scaling only once ?

I have no idea how images were rendered? loaded into memory? scaled etc ?

How does Sprite in libgdx work? I tried understanding the code of Sprite and looks to me like they are also getting image width and height and then scale it every time, even though they have setScale() method.

Answer 1

First rule of optimizing: get some numbers. Premature optimization is the root of many problems. That said, there are still some good rules of thumb to know.

The texture data will be uploaded by libgdx/OpenGL to the GPU when you invoke new Texture. When you actually draw the texture with spriteBatch.draw instructions are uploaded to the GPU by OpenGL that tell the hardware to use your existing texture and to fit it to the bounds. The draw call just uploads coordinates (the corners of the box that defines the Sprite) and a pointer to the texture. The actual texture data is not uploaded.

So, in practice your image is "scaled" on every frame. However, this is not that bad, as this is exactly what GPUs are designed to do very, very well. You only really need to worry about uploading so many textures that the GPU has trouble keeping track of them all, you do not need to worry much about scaling the textures beforehand.

The costs of scaling and transforming the four corners of the sprite are relatively trivial next to the costs of sending the data to the GPU and the cost of refreshing the screen, so they probably are not worth worrying about too much. The "batch" in SpriteBatch is all about "batching up" (or gathering together) a lot of coordinates to send up to the GPU at once, as roughly, each call out to the GPU can be expensive. So, its always good to do as much work within a single batch's begin/end as you can.

Again, though, modern machines are stupidly fast, and you should be able to do whatever is easiest to get your app running first. Then once you have something working correctly, you can figure out which parts are actually slow and fix those. The parts that are "inefficient" but are not actually measurably impacting your application can be left alone.